The 25-year-old Australian has become Instagram famous with her Bikini Body Guides. She also has an app that essentially puts her training in her fans' pockets.

The star has cultivated an online community that calls itself "Kayla's Army," largely by sharing impressive progress photos — or "transformations" — of the women who are committed to her program. It serves as visible inspiration.

Mallory Schlossberg contributed reporting on a previous version of this article.

The Bikini Body Guides are relatively simple.

Itsines provides her clients with three days of circuit-training routines that change every week. She encourages women to do sessions of "LISS" (low-intensity steady-state cardio) and later incorporate "HIIT" (high-intensity interval training) into their training regimen.

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The "HELP" — Healthy Eating and Lifestyle Plan — guide features two weeks of diet plans.

It's a 12-week program, and many women appear to repeat it. Itsines also offers a "2.0" version for people who finish the first guide and want more of a challenge.

Itsines is such a big hit on Instagram that even Victoria's Secret model Candice Swanepoel follows her.

"Yes! Candice started following me, which is amazing," she said to Shape. "I think that she is gorgeous. It was so good to see that a supermodel found my Instagram or program of interest."

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But Itsines didn't intend to become so popular.

"It was kind of a fluke," she told The Cut. "I needed somewhere to put my photos. I've been a personal trainer since I was 18, 19 years old. I'd go to people's houses and say, 'Oh, look at my client's transformation.' I'd scroll up my phone and then say, 'I can't find it I'll show you later.' My little cousin said, 'Why don't you just download Instagram and just start uploading new stuff on there?' She didn't really explain it properly, so I downloaded it and started documenting transformations. Then I realized that it doesn't work like Facebook. Everyone could see what I was doing."

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Itsines' brand, though, is not about her — it's about her clients.

"Although it's called Kayla Itsines, it's not me. It's not just my face and my brand. The brand is the transformations of women everywhere," she told The Cut. "People are drawn because it's a community and girls from all over the world. They can relate. If you're in this like-minded community, no one is going to say anything but positive stuff to you."

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Some of the hashtags these women use to share their journeys are #BBG, #TheKaylaMovement, and #KaylasArmy. But the community extends beyond the internet.

There are community groups across the globe that coordinate meetups. Arguably, Itsines' biggest accomplishment is that she has gotten millions of women to love exercising.

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Her app, Sweat with Kayla, is her latest venture.

"It's like I'm there with you every single day, showing you what food you need, your shopping list, what you need to do to work out, what rehabilitation you need," she told StyleCaster.

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It has come under fire, though.

Itsines' website promises that users will "be in the best shape, ever!" That's a lofty promise.

Further, it's understandable that some people might consider her claims to be dubious. A Reddit thread asks "yay or nay?" Many fitness bloggers have reviewed the program as though to test whether the BBG magic can work.

But the photos that Itsines shares seem to tell a different story.

Additionally, Itsines — who has debunked "30-second transformations" — picks the photos from Instagram accounts that she finds. They're not necessarily business affiliates, but some bloggers are affiliates.

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That said, the focus on obtaining a so-called bikini body could garner criticism — especially in this age of body positivity.

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Itsines swears that she's not promoting thinness.

"It's not about a certain body type," she said to The Cut. "The whole point of this program is to get rid of the 'skinny is better' thought process and focus on happiness and confidence. The plan gets harder and harder to push you mentally to do something that you haven't done before. It's not to make you smaller or to work your body harder. You can go at whatever pace you want to go. You finish the workout and you say, 'I did that.' It's fantastic for people who don't want to spend hours and hours in the gym. No one can go to the gym for two hours."

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And though she's steadily cultivating her own fitness empire, she doesn't think of it that way.

"I don't feel like it's a business," Itsines said to StyleCaster. "It's all happened so quickly and it still to me feels like a community. I'm not at the top, everyone's not looking up to me—we're growing together, like a family."